U.S. patent number 8,026,715 [Application Number 12/245,243] was granted by the patent office on 2011-09-27 for magneto-resistance based nano-scale position sensor.
This patent grant is currently assigned to International Business Machines Corporation. Invention is credited to Charalampos Pozidis, Deepak Ranjan Sahoo, Abu Sebastian.
United States Patent |
8,026,715 |
Pozidis , et al. |
September 27, 2011 |
Magneto-resistance based nano-scale position sensor
Abstract
A position sensor and method include a magnetic component, a
first magneto-resistive sensor disposed in proximity to the
magnet/coil; and a second magneto-resistive sensor disposed in
proximity to the magnetic component and the first magneto-resistive
sensor. The first magneto-resistive sensor and second
magneto-resistive sensor are configured to sense changes in a stray
magnetic field created by the magnetic component in accordance with
a relative positional change between the magnetic component and the
first and second magneto-resistive sensors.
Inventors: |
Pozidis; Charalampos (Thalwil,
CH), Sahoo; Deepak Ranjan (Zurich, CH),
Sebastian; Abu (Adliswil, CH) |
Assignee: |
International Business Machines
Corporation (Armonk, NY)
|
Family
ID: |
42075281 |
Appl.
No.: |
12/245,243 |
Filed: |
October 3, 2008 |
Prior Publication Data
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|
|
Document
Identifier |
Publication Date |
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US 20100085041 A1 |
Apr 8, 2010 |
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Current U.S.
Class: |
324/207.21;
324/260 |
Current CPC
Class: |
G01R
33/093 (20130101); G01D 5/145 (20130101); B82Y
25/00 (20130101); B82Y 15/00 (20130101); B82Y
35/00 (20130101) |
Current International
Class: |
G01R
33/06 (20060101); G01R 33/00 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
H Li et al.; Microelectromechanical System Microbridge deflection
monitoring using Integrated Spin Valve Sensors and Micromagnets;
Journal of Applied Physics; vol. 91, No. 10 May 15, 2002; pp.
7774-7776. cited by other .
T. Takezaki et al.; Magnetic Field Measurement Using Scanning
Magnetoresistance Microscope with Spin-Valve Sensor; Japanese
Journal of Applied Physics; vol. 45. No. 38; 2006; pp. 2251-2254.
cited by other .
M. Nakamura et al.; Scanning Magnetoresistance Microscopy with a
Magnetoresistive Sensor Cantilever; Applied Physics Letters, vol.
80, No. 15; Apr. 15, 2002; pp. 2713-2715. cited by other.
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Primary Examiner: Hollington; Jermele M
Attorney, Agent or Firm: Tutunjian & Bitetto, P.C.
Kaufman; Stephen C.
Claims
What is claimed is:
1. A position sensor, comprising: a magnetic component; a first
magneto-resistive sensor disposed in proximity to the magnetic
component; and a second magneto-resistive sensor disposed in
proximity to the magnetic component and the first magneto-resistive
sensor wherein the first magneto-resistive sensor and second
magneto-resistive sensor are configured to sense changes, with a
sub-nanoscale resolution, in a stray magnetic field created by the
magnetic component in accordance with a relative positional change
between the magnetic component and the first and second
magneto-resistive sensors.
2. The sensor as recited in claim 1, wherein the first
magneto-resistive sensor and the second magneto-resistive sensor
are attached to a same repositionable platform which is moveable
relative to a position of the magnetic component.
3. The sensor as recited in claim 1, wherein the magnetic component
is attached to a repositionable platform which is moveable relative
to both the first magneto-resistive sensor and the second
magneto-resistive sensor.
4. The sensor as recited in claim 1, wherein a position is measured
by a differential signal which determines a difference between an
output of the first magneto-resistive sensor and an output of the
second magneto-resistive sensor.
5. The sensor as recited in claim 1, wherein the first
magneto-resistive sensor and the second magneto-resistive sensor
are arranged in a first direction having a gap between the first
magneto-resistive sensor and the second magneto-resistive
sensor.
6. The sensor as recited in claim 5, wherein a position measurement
is made along the first direction.
7. The sensor as recited in claim 1, wherein the first
magneto-resistive sensor and the second magneto-resistive sensor
include at least one of Anisotropic Magneto -Resistive (AMR)
sensor, a Giant Magneto-Resistive (GMR) sensor, and a Tunnel
Magneto-Resistive (TMR) sensor.
8. The sensor as recited in claim 1, wherein the first
magneto-resistive sensor and the second magneto-resistive sensor
include micro-electro-mechanical-systems (MEMS).
9. The sensor as recited in claim 1, wherein the magnetic component
includes at least one of a permanent magnet, a magnetic layer and a
current-loop.
10. A position sensor, comprising: a magnetic component; a first
magneto-resistive sensor disposed in proximity of the magnetic
component; and a second magneto-resistive sensor disposed in
proximity to the magnet and the first magneto-resistive sensor
wherein the first magneto-resistive sensor and the second
magneto-resistive sensor are arranged in a first direction having a
gap between the first magneto-resistive sensor and the second
magneto-resistive sensor where a position measurement is made along
the first direction; the first magneto-resistive sensor and second
magneto-resistive sensor are configured to sense changes in a stray
magnetic field created by the magnetic component in accordance with
a relative positional change between the magnetic component and the
first and second magneto-resistive sensors in the first direction
such that a position is measured by a differential signal which
determines a difference between an output of the first
magneto-resistive sensor and an output of the second
magneto-resistive sensor.
11. The sensor as recited in claim 10, wherein the first
magneto-resistive sensor and the second magneto-resistive sensor
are attached to a same repositionable platform which is moveable
relative to a position of the magnetic component.
12. The sensor as recited in claim 10, wherein the magnetic
component is attached to a repositionable platform which is
moveable relative to both the first magneto-resistive sensor and
the second magneto-resistive sensor.
13. The sensor as recited in claim 10, wherein the first
magneto-resistive sensor and the second magneto-resistive sensor
include at least one of Anisotropic Magneto -Resistive (AMR)
sensor, a Giant Magneto-Resistive (GMR) sensor, and a Tunnel
Magneto-Resistive (TMR) sensor.
14. The sensor as recited in claim 10, wherein the magnetic
component includes at least one of a permanent magnet, a magnetic
layer and a current-loop.
15. A method for position sensing, comprising: providing a fixed
component attached to a reference position, the fixed component
including one of a magnetic component and a pair of
magneto-resistive sensors; providing a positionable component on a
movable platform, the positionable component including the other of
the magnetic component and the pair of magneto-resistive sensors;
sensing changes in a stray magnetic field created by a positional
change between the fixed component and the positionable component;
and subtracting the sensed changes of one of the pair of
magneto-resistive sensors from sensed changes of the other of the
pair of magneto-resistive sensors to measure the positional
change.
16. The method as recited in claim 15, further comprising: aligning
the pair of magneto-resistive sensors along a first direction where
the positional change is to be measured; and disposing the pair of
magneto-resistive sensors apart by a distance to form a gap
therebetween.
17. The method as recited in claim 15, wherein the subtracting
provides differentional configuration.
18. The method as recited in claim 15, wherein the pair of
magneto-resistive sensors include at least one of Anisotropic
Magneto-Resistive (AMR) sensor, a Giant Magneto-Resistive (GMR)
sensor, and a Tunnel Magneto-Resistive (TMR) sensor.
19. The method as recited in claim 15, wherein sensing changes
includes sensing changes with a sub-nanoscale resolution and a
bandwidth exceeding 1 MHz.
Description
RELATED APPLICATION INFORMATION
The present application is related to U.S. application Ser. No.
12/245,171, entitled "MAGNETO-RESISTANCE BASED TOPOGRAPHY SENSING",
filed currently herewith and incorporated herein by reference.
BACKGROUND
1. Technical Field
The present invention relates to position sensing, and more
particularly to a system, device and method for sub-nanometer
resolution position sensing which employs magneto-resistance.
2. Description of the Related Art
Position sensing with sub-nanometer resolution and high bandwidth
in moving structures like piezo-scanners, flexure stages and Micro
Electro Mechanical Systems (MEMS) based micro-scanners is important
for closed-loop controlled operation to ensure positioning accuracy
at very high speeds. Such moving stages are employed in scanning
probe microscopes, nano-lithography tools, nanoscale data storage
devices and experimental (probe-based) nano-fabrication tools.
Currently available position sensors based on optics, capacitors
and inductive coils (for example, linear variable displacement
transducers (LVDTs)), although accurate and fast, do not scale down
(with respect to the dimension of the sensor) to micro-scales for
use in micro-structures or in large-scale point-wise position
sensing of macro-structures. Thermo-electric and piezo-resistive
position sensors, on the other hand, easily scale down to
micro-scale, but suffer from low bandwidth.
SUMMARY
A position sensor and method include a magnetic component, a first
magneto-resistive sensor disposed in proximity to the magnet/coil;
and a second magneto-resistive sensor disposed in proximity to the
magnetic component and the first magneto-resistive sensor. The
first magneto-resistive sensor and second magneto-resistive sensor
are configured to sense changes in a stray magnetic field created
by the magnetic component in accordance with a relative positional
change between the magnetic component and the first and second
magneto-resistive sensors.
A method for position sensing includes providing a fixed component
attached to a reference position, the fixed component including one
of a magnetic component and a pair of magneto-resistive sensors. A
positionable component is provided on a movable platform, the
positionable component including the other of the magnetic
component and the pair of magneto-resistive sensors. Changes in a
stray magnetic field created by a positional change are sensed
between the fixed component and the positionable component, and the
changes in the stray magnetic field are associated to measure the
positional change.
These and other features and advantages will become apparent from
the following detailed description of illustrative embodiments
thereof, which is to be read in connection with the accompanying
drawings.
BRIEF DESCRIPTION OF DRAWINGS
The disclosure will provide details in the following description of
preferred embodiments with reference to the following figures
wherein:
FIGS. 1A-B are cross-sectional views showing a magnet and a pair of
magneto-resistive sensors (located on a platform) for sensing
positional changes of the platform in accordance with the present
principles;
FIG. 2 is a perspective view of a scan table showing a magnet
(located on a scan table) and a pair of sensors operatively
disposed relative to the scan table in accordance with one
illustrative embodiment; and
FIG. 3 is a flow diagram showing a method for making positional
measurements in accordance with the present principles.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The present principles provide new techniques for position sensing
with high bandwidth and high resolution. The present principles
provide the capability to include integrated sensors. The sensors
can be fabricated by e.g., sputter depositing process on to a
micro-structure. Particularly useful embodiments offer the combined
potential for high bandwidth, well in excess of 1 MHz, and high
resolution at the sub-nanometer level.
A position sensing system and method in accordance with the present
principles combines sub-nanometer resolution, bandwidth in excess
of 1 MHz, and scalability down to the micrometer-scale. The sensor
may be fabricated using existing MEMS fabrication processes. As
such, the sensor can also be used in various array configurations
for point-wise local position sensing in macro-structures.
Embodiments of the present invention can take the form of an
entirely hardware embodiment, an entirely software embodiment or an
embodiment including both hardware and software elements. In a
preferred embodiment, the present invention is implemented in
hardware with software support. The software may include but is not
limited to firmware, resident software, microcode, etc.
Furthermore, aspects of the invention can take the form of a
computer program product accessible from a computer-usable or
computer-readable medium providing program code for use by or in
connection with a computer or any instruction execution system. For
the purposes of this description, a computer-usable or computer
readable medium can be any apparatus that may include, store,
communicate, propagate, or transport the program for use by or in
connection with the instruction execution system, apparatus, or
device. The medium can be an electronic, magnetic, optical,
electromagnetic, infrared, or semiconductor system (or apparatus or
device) or a propagation medium. Examples of a computer-readable
medium include a semiconductor or solid state memory, magnetic
tape, a removable computer diskette, a random access memory (RAM),
a read-only memory (ROM), a rigid magnetic disk and an optical
disk. Current examples of optical disks include compact disk--read
only memory (CD-ROM), compact disk--read/write (CD-R/W) and
DVD.
A data processing system suitable for storing and/or executing
program code may include at least one processor coupled directly or
indirectly to memory elements through a system bus. The memory
elements can include local memory employed during actual execution
of the program code, bulk storage, and cache memories which provide
temporary storage of at least some program code to reduce the
number of times code is retrieved from bulk storage during
execution. Input/output or I/O devices (including but not limited
to keyboards, displays, pointing devices, etc.) may be coupled to
the system either directly or through intervening I/O
controllers.
Network adapters may also be coupled to the system to enable the
data processing system to become coupled to other data processing
systems or remote printers or storage devices through intervening
private or public networks. Modems, cable modem and Ethernet cards
are just a few of the currently available types of network
adapters.
The sensors as described herein may include of be part of the
design for an integrated circuit chip. The chip design may be
created in a graphical computer programming language, and stored in
a computer storage medium (such as a disk, tape, physical hard
drive, or virtual hard drive such as in a storage access network).
If the designer does not fabricate chips or the photolithographic
masks used to fabricate chips, the designer transmits the resulting
design by physical means (e.g., by providing a copy of the storage
medium storing the design) or electronically (e.g., through the
Internet) to such entities, directly or indirectly. The stored
design is then converted into the appropriate format (e.g., Graphic
Data System II (GDSII)) for the fabrication of photolithographic
masks, which typically include multiple copies of the chip design
in question that are to be formed on a wafer. The photolithographic
masks are utilized to define areas of the wafer (and/or the layers
thereon) to be etched or otherwise processed.
The resulting integrated circuit chips can be distributed by the
fabricator in raw wafer form (that is, as a single wafer that has
multiple unpackaged chips), as a bare die, or in a packaged form.
In the latter case the chip is mounted in a single chip package
(such as a plastic carrier, with leads that are affixed to a
motherboard or other higher level carrier) or in a multichip
package (such as a ceramic carrier that has either or both surface
interconnections or buried interconnections). In any case the chip
is then integrated with other chips, discrete circuit elements,
and/or other signal processing devices as part of either (a) an
intermediate product, such as a motherboard, or (b) an end product.
The end product can be any product that includes integrated circuit
chips, ranging from toys and other low-end applications to advanced
computer products having a display, a keyboard or other input
device, and a central processor.
Referring now to the drawings in which like numerals represent the
same or similar elements and initially to FIGS. 1A and 1B, an
illustrative sensing device 100 is illustratively shown. Sensing
device 100 is employed to measure position of non-magnetic
platforms although magnetic platforms may be measured as well.
Sensing device 100 employs a magnetic component 102 although any
device such as a permanent magnet, a current loop and a magnetic
layer capable of producing a stray magnetic field can be utilized.
Sensing device 100 is based on magneto-resistive (MR) sensing which
is used for measuring stray magnetic fields. MR sensor/device 100
provides both high bandwidth (e.g., greater than 1 MHz) and high
resolution (e.g., less than 1 nm).
Device 100 may include a plurality of different configurations
where a magnetic component (hereinafter magnet for simplicity) 102
and magneto resistive (MR) sensors 104 are employed, but may have
their locations transposed or their locations may be altered or
integrated into different components of the designs. For example,
FIG. 1A depicts a magnet 102 fixed at a reference position and two
MR sensors 104a and 104b which are mounted to a platform 105. FIG.
1B, depicts magnet 102 mounted on the platform 105 and the two MR
sensors 104a and 104b are fixed to a reference position. The
embodiments depicted in FIGS. 1A and 1B will be employed for
illustrative purposes.
It should be understood that magnet 102 may include a magnetic
field or any device that generates a magnetic field. For example,
magnet 102 may include a permanent magnet, a magnetic layer, a
current loop or coil, an inductor, etc.
Magneto-resistive (MR) sensing may employ any sensor 104 belonging
to the MR sensing family, e.g., Anisotropic Magneto-Resistive
(AMR), Giant Magneto-Resistive (GMR), Tunnel Magneto-Resistive
(TMR), etc.). These MR sensors 104 may be employed to probe a stray
field 112 of a magnet in different architectures for position
sensing.
Position variations between the magnet 102 and sensors 104a and
104b induce modulation of the magnetic field which is sensed by
sensors 104a and 104b. For example, platform 105 may be moved in
the x, y, z directions (or combinations thereof) to create
variations in the magnetic field 112. FIGS. 1A and 1B depict a
change in only the x direction to simplify the explanation.
When platform 105 moves in the x direction relative to the fixed
component (either the magnet 102 or the sensors 104a, 104b), there
is a change in a magnetic field (indicated by stray-field vectors,
V.sub.+.DELTA.x and V.sub.-.DELTA.x, associated with sensors 104a
and 104b, respectively). The magnetic field strength and
orientation are preferable selected in a strategic way to ensure
the greatest sensitivity and therefore the most accurate position
sensing.
As the platform 105 moves, the magnetic field through the sensors
104a and 104b changes and a position sensing signal is generated.
The position sensing signal is preferably a differential signal
which subtracts the measured field between the two sensors 104a and
104b (e.g., V.sub.+.DELTA.x-V.sub.-.DELTA.x). The differential
configuration assists in accounting for drift, rejects effects from
stray magnetic fields external to the device and rejects resistance
variations due to temperature fluctuations. To counter the effects
of drift, stray magnetic fields, and to minimize resistance
fluctuations on the MR sensors due to ambient temperature changes,
two or more MR sensors are employed in the differential
configuration. In this way, as both sensors are placed nearby,
rejection of drift and fluctuations as well as stray fields common
to both sensors can be achieved, and a signal to noise ratio (SNR)
of the position signal can be enhanced. Since both sensors 104a and
104b experience the same conditions, the subtraction results in the
elimination of such effects. In this case, the difference results
in Ax or the positional change in the x direction.
Referring to FIG. 2, a perspective view of a scan table 202 is
depicted to demonstrate a particularly useful embodiment in
accordance with the present principles. Scan table 202 may be
employed in a manufacturing application, on a microscope, an
inspection station, alignment device, etc. The illustrative
application depicts motion in a single dimension 204. It should be
noted that magnet 102 and MR sensors 104a and 104b may be employed
for motion in more than one dimension. In one particularly useful
embodiment, a set of sensors 104a and 104b and magnet 102 may be
orientated for each dimension for which position is to be
measured.
One of the magnet 102 or the sensors 104a and 104b is attached to
and moves with the scan table 202 (e.g., platform 105). The other
of the magnet 102 or the sensors 104a and 104b is attached to a
rigid frame 206 adjacent to the scan table 202. The rigid frame 206
acts as a reference position from which sensors 104a and 104b can
make relative measurements from. The sensors 104a and 104b include
a gap 210 therebetween that is aligned with the sensors 104a and
104b in the direction of motion 204.
When the scan table 202, whose position is to be measured, moves,
the motion induces a change in the magnetic field of the magnet 102
passing through the MR sensors 104a and 104b due to the relative
movement of the MR sensor 104a, 104b with respect to the stray
magnetic field from the magnet 102.
If the sensors 104a and 104b are mounted on the scan table 202, a
circuit or circuit connections are needed to measure and process
output signals from the sensors. This may include making
connections to the sensors located on the scan table through legs
(not shown) which connect the scan table to the rigid frame or
employing a printed circuit board or a chip 212 on the scan table
202 or employing the board or chip 212 on the rigid frame depending
on the configuration. The board or chip 212 may be connected to
data acquisition software/equipment, positioning
equipment/software, etc. The software may be run on a processing
device which may include but is not limited to a computer device or
system. The output signals are interpreted and employed as feedback
for positioning the scan table 202 or simply for measuring between
locations of interest on the scan table 202. Position data may be
collected and recorded.
A type of magneto-resistive sensor, e.g., a giant magneto resistive
(GMR) sensor, includes a stack having anti-Ferro magnetically
pinned layers and soft magnetic free layers having conductive
nonmagnetic interlayers. In a high resistance state, e.g., in the
absence of an external magnetic field, the magnetic moment in the
two magnetic layers is opposite to each other due to ferromagnetic
coupling. In the presence of an external magnetic field, the
magnetic moment of the magnetically free layer aligns itself in the
direction of the external magnetic field by overcoming the
anti-Ferro magnetic coupling. Due to interfacial spin-polarized
scattering between the ferromagnetic layers separated by conductive
layers the electrical resistance of the sensor changes. The
resistance varies as a cosine function of the angle between the
magnetic moments of the pinned layer and the free layer. The
thinner the layers, the higher the resistance change is. A maximum
resistance change of a GMR sensor is between about 10% and about
20% and can be as high as 110% at room temperature.
Dimensions of the permanent magnet (or other means for generating
stray magnetic field) should be comparable with dimensions of the
magneto-resistive sensor so that when the platform moves along any
axis, the stray magnetic field through the magneto-resistive sensor
changes by different amounts along a sensing direction. The
magnetic moment of the soft layer of the sensor aligns itself along
the component of the stray magnetic field in its plane and the
resistance of the sensor changes. A constant current is passed
through the sensor and voltage output from the sensor is used as an
imaging signal.
The sensitivity and resolution of position sensing scales
proportionally to the sensitivity and resolution of the
magneto-resistive sensor. The sensitivity of the method is improved
by tuning the spatial distribution of the stray magnetic field
through the magneto resistive sensor to exploit its full range of
operation of the MR sensor.
The spatial distribution of the stray magnetic field is not linear
for large ranges of operation and depends on the size of the
magnet. However, for all practical purposes the MR sensor signal
can be assumed a linear function of the position of platform. For
large ranges of motion, mapping between the MR sensor signal and
the position of platform is a static nonlinear map which can be
used to interpret the MR sensor signal. This method senses the
movement of the positioning stages similar to methods including
optical, capacitive, LVDT and piezo-resistive methods and may
utilize signal processing techniques used in those methods. In
particular, for high speed measurements, high bandwidth, low noise
electronics is needed which should also have a good gain to achieve
good sensitivity in measurement.
The sensitivity/resolution may be optimized by trial and error, by
computation, by design, by experience or combinations thereof.
Magneto resistive sensors usually operate at low field strengths
starting from zero Oersted to few hundreds of Oersted. At higher
field strength the soft magnetic layers get saturated and the
sensor loses sensitivity. The stray magnetic field at the MR sensor
in its sensitive direction can be oriented by carefully choosing
the shape, size and material of a permanent magnet, and the
relative position of the magnet with respect to the MR sensor. The
stray magnetic field at the MR sensor can also be oriented by using
combinations of more than one permanent magnets, magnetic layers
and current loops. Magnetism simulation tools can be utilized to
simulate various configurations and compute the distribution of the
optimal stray magnetic field for sensing.
With careful topological placement of the MR sensors 104 and magnet
102, and through miniaturization, a high bandwidth and high
resolution position sensing signal can be obtained. The position
sensors 100 can be operated in parallel in an array configuration
for local, point-wise motion measurement of elements of larger
structures.
The present principles provide substantial advantages over the
known solutions in position sensing. For example, very high
bandwidth is achieved at low-cost and with miniature form-factors.
This is in contrast to the bulky and expensive optical, capacitive
and inductive-coil conventional setups. Very high resolution is
also achieved. The resolution of the magnetic sensing scheme can
theoretically match sub-nanometer resolution of
optical/capacitive/LVDT sensing, by appropriate placement of
sensors and magnet and miniaturization of both. The potential for
MEMS fabrication is available, which is a low cost fabrication
technique. The magneto-resistive position sensing in accordance
with the present principles advantageously combines the small
form-factor and integrated fabrication capability of
thereto-electric sensing, as well as the superb
bandwidth/resolution performance of optical/capacitive sensing.
Referring to FIG. 3, a method for position sensing is
illustratively depicted. In block 302, a fixed component is
attached to a reference position. The fixed component includes one
of a magnet and a pair of magneto-resistive sensors. In block 304,
a positionable component is provided on a movable platform, such
as, e.g., a scan table or the like. The positionable component
includes the other of the magnet and the pair of magneto-resistive
sensors. The pair of magneto-resistive sensors may include one or
more of an Anisotropic Magneto-Resistive WHO sensor, a Giant
Magneto-Resistive (GMR) sensor, and a Tunnel Magneto-Resistive
(TMR) sensor.
In block 306, the pair of magneto-resistive sensors is aligned
along a first direction where the positional change is to be
measured, and the pair of magneto-resistive sensors are disposed
apart by a distance to form a gap therebetween. In block 308,
changes in a stray magnetic field created by a positional change
between the fixed component and the positionable component are
sensed by the sensors. Changes in position can be determined with a
sub-nanoscale resolution.
In block 310, the sensed changes in the stray magnetic field are
associated between sensors to measure the positional change. This
preferably includes subtracting sensed changes from one of the pair
of magneto-resistive sensors from the other of the pair of
magneto-resistive sensors to provide a differential configuration
in block 312.
Having described preferred embodiments of a system and method for a
magneto-resistance based nano-scale position sensor (which are
intended to be illustrative and not limiting), it is noted that
modifications and variations can be made by persons skilled in the
art in light of the above teachings. It is therefore to be
understood that changes may be made in the particular embodiments
disclosed which are within the scope and spirit of the invention as
outlined by the appended claims. Having thus described aspects of
the invention, with the details and particularity required by the
patent laws, what is claimed and desired protected by Letters
Patent is set forth in the appended claims.
* * * * *